It is an improved version of the 5T1. The job of the 5T1A is to produce three signals:
the sampling pulse, the horizontal sweep, and the blanking signal.
The input to the 5T1A is a trigger signal. The trigger can arrive
from any of four sources:

from the calibration oscillator in the 661, via the multi-pin plug-in connector

free-running

Each of these modes is suited to some measurement scenarios.
In the case of internal triggering, the signal goes from the trigger pick-off
in the 4S1 to a coaxial interconnect that passes
through the 661, connecting the sampling unit to the timing unit.